It only takes 1 miner and 1 user to fork the blockchain and create an alternative Bitcoin. It does not require 51% of the miners.
On the other hand, it requires 100% of all users (not 51% of the miners) to change anything in the fundamental nature of bitcoin.
You clearly have misunderstood what a 51% attack is.
Indeed, one can at anypoint fort the blockchain and create an altcoin with it's own transaction history diverging for the "real" bitcoin from that point on. However nobody would put much trust nor value in such alt chain.
If someday, more than half of the miners/hashpower agreed to change something fundamental about bitcoins, (say, remove the 21 million bitcoins cap). Of course, it would require an hard fork. Users who agree with the change will use the new blockchain, while those who dissagree will continue tu use (and mine on) the old block chain. But since both will diverge from that point on, new transactions valid on one wont on the other. Sooner or later, people will have to make a choice, wether to use the new or the old. And then, the "losing" chain's coins will be worthless. Don't forget that bitcoin's value is purely based on thrust that it can be exchanged for something. They have no inherent value.
So changing anything fundamental "only" requires a broad concensus. (if only half of people wanted the change, I guess both may exist side by side, but it would be extremely confusing.)